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Know the basics of video transcoding for CE

Posted: 16 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:video transcoding? CE? container formats? network protocols?

Over the past several years there has been a shift in how consumers access and use audio/video media content. In the past, the triple services; data, voice, and video have come from the traditional sources:

??Data through the PC via modems, traditionally plain old telephone service, but the trend has been DSL and cable modem;
??Voice over wired and wireless telephone;
??Video watched on TV with content provided by VCRs, DVD players, and cable or satellite STBs.

Today, consumers want to easily move content from device to device depending on the location where it will be used and to be able to do this quickly at no additional cost, often in real-time or even faster than real-time. In this dynamic ever changing environment, the devices available to the consumer, which are many, must be able to exchange the content that is stored or viewed on them such that they can be viewed on alternative devices at different times by potentially a different consumer, all within the confines of the home originally, but eventually taken outside the home.

Changing compression format
In making this a reality, there will be three necessary functions that will need to be supported. The first and most obvious is transcoding. This refers to the coding and recoding of digital A/V content from one compressed format to another to allow transmission and playback by various devices. This becomes increasingly important as high-definition (HD) video is more widely available and viewed by the consumer.

There are three main types of transcoding:

?? Transcoding (traditional reference)!involves the conversion from one compression format to another, such as when converting MPEG-2 to H.264. This method involves the most changes to the original content; codec tools, image size, frame rate, and bit rate.
??Transrating of content!using the same compression format, but lowering the bit rate of the original content to allow it to be transmitted, stored, or used by a less capable device.
??Transcoding of image parameters!using the same compression format (same profile, different level), but reducing the original image size and frame rate to allow for playback on less capable devices.

Transcoding results in reductions in HDD storage, bandwidth use, need for storing multiple files and need for supporting multiple formats. These reductions are further pronounced with HD where the savings from transcoding are very substantial and in some cases!such as with bandwidth-limited pipes!would enable capabilities that would otherwise be inoperable.

But it's not just about transcoding. There may be incompatibilities in container formats that devices use to store and transmit their A/V content. Even with proper transcoding, some devices may not be able to operate if they have incompatible container formats, even though the A/V media inside the container is compatible. Hence there needs to be a method to convert incompatible container formats among different devices to a common container format that one or the other can understand.

Incompatibilities in the transmission of the A/V media can also occur such that the network protocols supported by the different devices are incompatible. In particular, a media client that is connecting to a media server must be able to understand the format of the data that they send and receive. Obviously they each must support the same protocol (i.e. hypertext transfer protocol or HTTP, real time streaming protocol or RTSP etc.) and one or the other has to switch over (protocol roll over) to support a lowest common denominator of the available protocols of the two connecting devices. Yet even within the same protocol, there can still be incompatibilities where certain modes or features are not supported, preventing the content from being received and played back, resulting in a dissatisfied consumer.


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